1. Field of the Invention
The present invention relates to a technology for extracting ruled-line projection.
2. Description of the Related Art
Conventionally, in the financial industry, such as banks, a technique for inputting paper forms and paper documents at a high speed by a non-contact imaging device such as a charge coupled device (CCD) or a digital camera has been put to practical use to perform input operations efficiently. However, in an image input by the non-contact imaging device, there are various image distortions resulting from curvature of the paper (for example, see FIGS. 19 to 23 of Japanese Patent Application Laid-open No. 2002-150280).
For example, the present applicant has disclosed a technique for estimating a three-dimensional curved surface of paper face (hereinafter, “three-dimensional paper curved surface”) based on a distortion of a two-dimensional profile of the paper extracted from an image to correct the image distortion in Japanese Patent Application Laid-open No. 2002-150280.
In the conventional technique, the type of distortion to be corrected is limited, so that the type needs to be specified. Moreover, when an entire sheet of paper is not imaged in the captured image, the profile thereof cannot be extracted, and therefore applicable images are limited.
There is known a method for creating an image where image distortion is corrected by extracting a set of parallel geodesic projections from a focused image of the three-dimensional paper curved surface, to extract a set of ruled line projections by using such a property that paper is a ruled surface, estimating the three-dimensional paper curved surface from the set of parallel geodesic projections and the set of ruled line projections, and determining a pixel-to-pixel correspondence between the estimated three-dimensional paper curved surface and the focused image, as disclosed in “Image distortion-correcting program, image-distortion correcting apparatus, image distortion-correcting method, and recording medium storing image distortion-correcting program” (Japanese Patent No. 2005-514903), which is a prior invention of the present applicant.
In the above method, however, combinations of sets of ruled-line candidate projections searched for on the focused image are huge, and a huge calculation amount is required for calculating a sum total of the “deviation” for all cases. For example, even if the calculation amount is reduced by using a steepest descent method, which is a sequential search method, the set of ruled-line candidate projections that gives a local optimum value is extracted as the set of ruled line projections, depending on an initial value to be set. Accordingly, a highly accurate and stable set of ruled line projections cannot be extracted.
Further, according to the above method, the sum total of “deviation” can be calculated only for the ruled-line candidate projections, which intersect all of the sets of the extracted parallel geodesic projections, and “deviation” of the ruled-line candidate projections, which do not intersect all of the parallel geodesic projections, are excluded from the search target as the ruled line projection. Therefore, for example, there is a possibility that the sets of the ruled line projections are extracted mainly in a central partial set of parallel geodesic projections on the image. Accordingly, there is a problem in that the paper distortion cannot be corrected highly accurately and stably, since the set of ruled line projections cannot be extracted highly accurately and stably.